The present invention relates to a method and apparatus for transmission of alarm signals, and is based on utilization of an entrance telephone set system connected to a public automatic subscriber telephone network of the type disclosed in U.S. Pat. No. 3,947,641, or any other similar type of system, e.g. as disclosed in U.S. Pat. Nos. 3,917,911 or 4,113,986.
It is previously known to utilize a public subscriber telephone network for transmission of various types of alarm signals, either utilizing continuously switched-through connections, or utilizing mechanically or electronically operated automatic dialling equipment, during alarm condition arranged to facilitate connection via a telephone line to a predetermined subscriber telephone number, e.g. an alarm surveillance centre.
U.S. Pat. No. 3,947,641 discloses how a public telephone network can be used for entrance signalling and door opening purposes. By means of a telephone line from a building, specific entrance calls are switched via the public telephone network to tenants within the building. This is made possible by the use of a device, referred to as "the Number Discriminator", restricting the use to preprogrammed, but otherwize conventional telephone numbers. A common "automatic dialler", which can only dial one or a number of certain preprogrammed telephone numbers, acts as such a discriminator, since calls can only be made to parties dialled by the automatic dialler. However, this is only true, provided that no other conventional methods for dialling are offered to the calling party. A device as indicated above could be denominated as an "active" number discriminator, which by design and programming has been preset with regard to which telephone number to be dialled, based on the way in which the device is addressed. The telephone number dialler is thus only accessable for a visitor by means of a conventional name/signalling panel. When the signalling key for a certain tenant is depressed, or by short number or telephone subscriber number selection from a key set, an "entrance call" is initiated.